Tuesday, November 20, 2007

Greenhouse - Indicator Plants, Trap Crops, and Banker Plants

Indicator plants, trap crops, and banker plants are biological plant based tools to monitor insect and mite pests, aid in control of these pests, and to maintain biocontrol species. The following is an article on the subject.

Indicator Plants: The diversity of plant species in an area can be used to indicate something about the environment: ground ivy in shady lawns, rhododendrons in acid soil, etc. In the same way, species or varieties that are more susceptible to an insect or disease than the desired crop can be used to indicate when that pest has appeared in the greenhouse. Some common greenhouse examples are potato chunks used to check for fungus gnat larvae (not quite an indicator plant, but the same principle) or specific cultivars of petunias grown as indicator plants for impatiens necrotic virus or tomato spotted wilt virus on a variety of ornamentals. Tomato or eggplant can be used as an indicator of whitefly infestation in poinsettia crops. It is important that the disease or insect be identified rapidly on the indicator plant, before it moves onto the crop, so scouting is still essential when using indicator plants.

Trap crops are also species or cultivars that are more attractive to a pest than the crop, but in trap cropping, the pest is controlled on the trap crop. This system has most often been used in field crops for insect control, where the trap crop is planted around the perimeter of the crop plant to attract insects moving in from outside the field. Some examples are: collards as a trap crop for diamond back moth in cabbage, Hubbard squash for cucumber beetles in other Cucurbitaceous plants, and cherry peppers for pepper weevil in bell peppers. Once the insect is found on the trap crop, only those plants are treated, reducing the amount of chemical pesticide needed. Perimeter trap crops can be used in the greenhouse, but it is more useful to intersperse the trap crop in the desired crop, as it is less likely that insects will be coming into the crop from outside. A set of plants could be positioned near wall vents to monitor insects coming in from outside the greenhouse, however. In addition to tomato or eggplant in poinsettia, gerbera or verbena or a more susceptible cultivar of chrysanthemum have been used to protect chrysanthemum from western flower thrips. The trap crop can be treated before or after it goes into the greenhouse, depending on the chemicals available for use.

Banker plants take the same concept one step further for biological control in greenhouse crops. In this case, the banker plant is used to rear insects that act as an alternative food source for the biocontrol agent, to reduce dramatic changes in its population. The most commonly used banker plant system is a grass, such as wheat or barley, infested with a grass-preferring aphid species, such as bird cherry or corn leaf aphid, to control melon or green peach aphid in ornamental or vegetable crops. The predators or parasitoids are very mobile and can use either the pest population on the crop or on the banker plant as host. There is interest in using the pest population itself as part of a banker plant system, by using the more susceptible cultivar or species, like the indicator or trap crop, as a banker plant. Because the banker plant is infested earlier, predators and parasitoids could colonize it and the population of biocontrol agents would increase before the crop was infested.

Reprinted from "Indicator plants, Trap crops, and Banker plants: Tools for Greenhouse IPM" in the December 2006 edition of the Ornamental Crops IPM E-newsletter from Cornell University.

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